1. Field of the Invention
The disclosures herein generally relate to an image forming method, an image forming apparatus, and an image forming program for forming an image by an inkjet method.
2. Description of the Related Art
An inkjet printing method has advantages such as high-speed printing, the ability to print on a normal paper sheet without requiring a special fusing process, and a small level of noise produced at the time of printing. Because of this, the inkjet printing method has been attracting attention for use in office. Various types of inkjet printing methods have been studied and put into practical use as commercial products. These inkjet printing methods use a print head that includes an ink liquid chamber and a nozzle communicating with the chamber. A pressure is applied to ink in the ink liquid chamber in response to image information or the like, so that an ink droplet is sprayed through the nozzle to be attached onto a print sheet such as a paper sheet or film to form an image.
An image forming apparatus (e.g., inkjet printer) using such an inkjet printing method can print on various types of print media because ink is sprayed from the print head to form an image in a non-contacting manner. Inkjet printers are classified mainly into a serial type and a line type.
The serial-type inkjet printer forms an image by moving a print head back and forth in a main scan direction perpendicular to a sheet travel direction (i.e., a sub-scan direction). The line-type inkjet printer has a print head fixedly arranged along the extension of a sheet width to form an image. The serial-type inkjet printer and the line-type inkjet printer have a common problem in that streaks or uneven appearance may appear.
A serial-type inkjet printer forms an image while moving a print sheet. As a result, a streak or uneven appearance may appear in the image at the boundary between adjacent scan lines due to various reasons such as sheet movement error, angular displacement of the print head, etc. There is also a serial-type inkjet printer in which a plurality of print heads is connected in series to provide an elongated print head for increasing a print speed. An inkjet printer having such an elongated print head may suffer the problem of a streak or uneven appearance at a boundary between print heads due to assembly error. The problem of a streak or uneven appearance caused by the error at the junction point between print heads is hard to solve because the assembly of the print heads fixes the positional relationships of the print heads, which makes it difficult to perform adjustment after assembly.
In a line-type inkjet printer, print heads are connected in series to provide a sufficient length to cover the width of a print sheet. In such a line-type inkjet printer, generally, the print heads are fixedly mounted, and perform one-path printing, which completes the formation of an image by a single scan of the heads. Accordingly, such measures as to reduce streaks through multiple scans of the heads cannot be adopted, so that the problem of streaks or uneven appearance is more pronounced.
In respect of the problem of a streak or uneven appearance at the seam line (i.e., one of the boundary between heads and the boundary between scan lines, which are essentially the same as a phenomenon appearing on a print sheet, and, thus, will not be discriminated from one another), there is a technology to reduce a streak or uneven appearance by providing nozzles overlapping each other. Nozzles situated at an end of a head is arranged to overlap nozzles of another head, and overlapping pixels are printed by a plurality of print heads, thereby reducing a streak or uneven appearance caused by density variation of the dots.
Japanese Patent Application Publication 2007-015180 discloses printing dots in the overlapping area selectively by the print heads in a staggered manner and also changes the dot size in the overlapping area in response to a displacement, thereby reducing streaks and uneven appearance. Japanese Patent Application Publication 2005-169628 discloses controlling the amount of ink ejected from the overlapping nozzles in response to information relating to the amount of ink printed in a predetermined area, thereby reducing streaks and uneven appearance.
The related-art technologies described above can reduce streaks or uneven appearance appearing in an image within the overlapping area, but have a problem in that a streak tends to conspicuously remain at the end of the overlapping area. When print heads are brought closer to each other in the sub-scan direction, for example, a black streak appears at the seam portion. The overlap processing as described can reduce the density variation of dots to lessen the black streak within the overlapping area. At the boarder between the overlapping area and the non-overlapping area, however, a high-dot-density portion is created, resulting in an area of high density being left to remain.
When print heads are taken apart from each other in the sub-scan direction, for example, a white streak appears at the seam portion. In this case, a low-dot-density portion is created at the border between the overlapping area and the non-overlapping area, resulting in an area of low density being left to remain. The related-art technologies previously described control ink ejection in the overlapping area. However, a main cause of a streak or uneven appearance resides at the border between the overlapping area and the non-overlapping area. Further, those related-art technologies are not based on simple processing configurations. Since several nozzles are used in the overlapping area, the control of the nozzles in the overlapping area results in an increase in processing load.
Accordingly, there is a need for an image forming method, an image forming apparatus, and an image forming program that can reduce a streak or the like appearing at the end of an overlapping area.